Common Air Conditioner Faults and How to Recognize Them

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Common air conditioning faults and how to recognize them

An air conditioner is a fairly reliable device – if properly designed, professionally installed, and regularly maintained, it can operate without problems for 15 to 20 years. Despite this, faults do occur. Some are trivial and can be handled by any user themselves, while others require the intervention of a certified F-gas service technician. The problem is that most users – whether in households or professional environments – cannot distinguish whether the air conditioner just needs its filters cleaned or if something serious has gone wrong in the refrigerant circuit. The result? Either they call for service unnecessarily (and pay for the visit), or they ignore the symptoms for too long, until a minor fault turns into an expensive breakdown.

In this article, we will systematically cover the most common air conditioning faults – what breaks down, why it breaks down, how to recognize it, and what you can do about it. We will focus on split and multi-split systems, which dominate the market today, including systems from the IVAR.2.0 range, with which we have extensive practical experience. If you are interested in proper regular maintenance, we recommend reading the article Air conditioning maintenance and service – what to do regularly and what to leave to a professional in parallel.

Basic logic: indoor vs. outdoor unit

Before we get into specific faults, it is important to understand that every split system has two parts – the indoor unit (evaporator with a fan and electronics) and the outdoor unit (compressor, condenser, expansion valve). These two parts are connected by copper pipes through which the refrigerant circulates. Electrical wiring provides power and communication.

Most faults can be categorized into one of three groups:

Mechanical faults – problems with the fan, compressor, condensate pump
Refrigerant circuit faults – refrigerant leak, clogged pipe, problems with the expansion valve
Electrical and control faults – sensor failures, problems with printed circuit boards, communication faults between units

Fault No. 1: The air conditioner does not cool (or heat) sufficiently

This is by far the most common problem we encounter. A customer calls saying that "the air conditioner does not cool well enough" – and there can be a dozen reasons. Let's go through them systematically, from the simplest to the most serious.

Clogged air filters

These account for about 40% of cases of weak cooling. The filter in the indoor unit captures dust, fibers, and other impurities. When it gets clogged, air passes through it with much greater resistance – the airflow through the heat exchanger drops, performance plummets, and the device works unnecessarily harder. In practice, we see filters that have not been cleaned for 2–3 years and are thick with dust like felt. Cleaning filters is a task for any user: simply remove them, shake off the dust, or wash them in warm water and let them dry. Clean the filters every 4–6 weeks during regular use, and in restaurants, production halls, or spaces with increased dust levels, every 2 weeks is fine.

Dirty heat exchanger (evaporator or condenser)

Filters capture coarse impurities, but fine dust, grease, and biological deposits gradually cover the fins of the heat exchanger itself. A dirty evaporator in the indoor unit loses the ability to efficiently absorb heat, while a dirty condenser in the outdoor unit cannot properly transfer heat to the outside environment. The result is the same in both cases: the device operates at higher pressures, the compressor overheats, and performance drops. Cleaning heat exchangers is part of annual service – for the indoor unit, there are special foams and preparations available, while the outdoor unit can be carefully rinsed with water (never directly at high pressure on the fins, which can easily bend).

Low refrigerant level – leak

If the filters and heat exchangers are clean, but the air conditioner still does not cool, a very likely cause is a lack of refrigerant. The refrigerant circuit is hermetically sealed – under normal conditions, refrigerant should not escape anywhere. If the level drops, there is a leak somewhere. Typical locations: pipe connections, valve stems, micro-cracks in the pipes (often after mechanical damage), corrosion damage to the heat exchanger. A refrigerant leak must not be handled by yourself – work with refrigerants is subject to strict legislation and requires certification under EU Regulation No. 517/2014 on fluorinated greenhouse gases. The technician will recharge the refrigerant only after locating and repairing the leak.

Compressor problems

The compressor is the heart of the entire refrigerant circuit. If it is working incorrectly or failing, the air conditioner does not cool at all or cools very little. Early signs of a compressor with problems: louder and unusual noise when starting, frequent shutdowns after a short period of operation, excessive vibration of the outdoor unit, error code on the display. A compressor can fail for several reasons – lack of refrigerant (operating at too low pressure), contaminated oil in the circuit, deep vacuum conditions, mechanical wear after years of operation. Replacing a compressor is expensive – for older devices, it is worth calculating whether it is worth it at all compared to buying a new unit.

Problem No. 2: Air conditioner dripping – excessive condensation

A dripping indoor unit is a problem customers react to quickly – after all, no one wants a wet ceiling or a wet wall. The causes are usually two: a clogged condensate drain, or ice buildup on the heat exchanger (and its subsequent melting).

Clogged condensate drainage system

During cooling, air humidity condenses on the cold heat exchanger of the indoor unit. This water drains out through a drain tray and a plastic hose – either through the wall directly outside, or into a sewer drain. This hose can become clogged with biological growths (mold, algae), deposits, or simply break mechanically if it was not properly installed with a slope. The result is an overflowing tray and water running down the indoor unit and the wall. Solution: clean the drain mechanically or with a gentle flush, or chemically. Always ensure during installation that the drain pipe has a minimum slope of 1–2 % outward and is not longer than the manufacturer recommends.

Frozen heat exchanger

Ice on the evaporator is always a sign of a problem – either weak air flow (clogged filters), or insufficient refrigerant, or too low room temperature during cooling. When the compressor is running, ice forms on the heat exchanger. When the compressor is turned off or defrosted, the ice melts and the water drains – or, if the tray is clogged, it runs directly down. If you see ice on the heat exchanger, immediately turn off the cooling and let the unit defrost with the fan running. Then look for the cause – in the vast majority of cases, it is either the filters or the refrigerant.

Problem No. 3: Air conditioner making unusual noises

Every air conditioner makes some noise during operation – the fan hum, slight clicking when starting and stopping (thermal expansion). However, when a new sound appears that wasn’t there before, it is always a signal. Here are the most common sound symptoms:

Squealing or buzzing from the fan – worn fan bearings or a foreign object in the fan wheel. A foreign object (e.g., an insect, leaves) is common in outdoor units, where it can be easily removed. Bearings require replacement of the fan motor.
Loud knocking or thumping – a loose fan blade, damaged fan wheel, or a serious problem in the compressor (e.g., a broken fin). This needs to be addressed immediately – further operation can cause extensive secondary damage.
Gurgling or bubbling – a typical sound of refrigerant leak. When refrigerant is low, air bubbles circulate in the pipes, producing a characteristic gurgling sound. Sometimes customers describe it as “as if water is running in the pipes.”
Loud vibrations from the outdoor unit – loose screws on the cover, worn rubber mounts under the compressor, or a compressor starting to fail. The source of vibrations can often be identified by touching different panels during operation (carefully!).
Clicking when trying to start – electrical problem: relay, start capacitor of the compressor, or a problem with the control board. The air conditioner tries to start but fails to do so.

Problem No. 4: Unpleasant odor from the air conditioner

An odor coming from the indoor unit is a problem that customers sometimes tolerate for a long time before reporting it. However, it can indicate rather serious issues.

Moldy or musty smell

By far the most common. The moist environment in the indoor unit is an ideal place for mold and bacteria to grow – on the fins of the heat exchanger, in the drain tray, in the drain hose. With each fan start, spores and bio-products are spread throughout the room. Solution: biological cleaning of the indoor unit with special agents (disinfecting foams, UV lamps), thorough cleaning of the drainage system. Preventively, let the air conditioner run for 15–20 minutes with only the fan after cooling has ended, to partially dry the heat exchanger and tray.

Chemical or sharp smell

May indicate a refrigerant leak (some types of refrigerants have a characteristic smell), problems with electrical components (burning insulation), or chemicals from the environment that have entered the unit. Never ignore a chemical smell – turn off the unit and call a service technician.

Smell of smoke or burning

Immediately turn off the air conditioner at the main circuit breaker. A smell of burning may indicate the beginning of insulation burning on electrical wires, a problem with the control board, or an overheating motor. This is a situation requiring immediate action.

Problem #5: The air conditioner does not start or turns off without reason

An air conditioner that does not start at all is frustrating – especially during a hot summer. But the causes can be surprisingly simple.

Electrical problems – circuit breaker tripping

The first thing to check: the circuit breakers in the electrical panel. The air conditioner usually has its own circuit breaker – check if it has tripped. If the circuit breaker repeatedly trips, it is a sign of a fault (short circuit, overload), and you should call an electrician. Do not repeatedly reset the circuit breaker manually without diagnosis.

Protection shutdown – device protection

Modern air conditioners have sophisticated protection: they shut down when the outside temperature is too low or too high, when the refrigerant pressure is too low or too high, when the compressor overheats, or when there is a communication issue between the units. If the device runs briefly and then stops on its own, it is almost always a protective function. The error code on the control panel display or in the app will tell you which protection was triggered.

Remote control issues

Surprisingly common. A dead battery in the remote control, a fault in the IR receiver on the indoor unit, or simply the device being in the wrong mode (e.g., the timer is on and blocks manual control). Always try to control the device directly using the buttons on the indoor unit, if available.

Communication fault (for multisplit systems)

In multisplit systems, where the outdoor unit serves multiple indoor units, communication between the units is critical. A fault in the communication cable or the control electronics can cause the entire system to refuse to start. Learn more about multisplit systems in the article Split vs. multisplit air conditioning – which is more cost-effective.

Problem #6: Issues with the outdoor unit – physical damage and blockages

The outdoor unit is exposed to weather all year round. Rain, frost, sun, and wind all leave their marks. In addition to natural wear and tear, the following problems can occur:

Clogged condenser fins

The fins of the condenser in the outdoor unit are designed for maximum heat transfer – they are thin and dense. That is why they easily catch leaves, spider webs, poplar fluff, insects, and various dirt. Clogged fins dramatically reduce cooling performance and cause the compressor to work under high pressure. Check the condition of the fins at least twice a year – in spring and autumn. To protect against insects, we recommend using THE INSECT PROTECTION KIT FOR IVAR.2.0 AIR CONDITIONERS, which prevents insects from entering the outdoor unit without restricting airflow.

Damage from rain and corrosion

Long-term exposure to rain, especially acidic rain in cities, accelerates the corrosion of the fins and metal grilles of the outdoor unit. Faults can also occur when heavy rain directly hits the fan inlet – water can reach the electrical components. For IVAR.2.0 outdoor units, there is a special RAIN PROTECTION KIT FOR IVAR.2.0 AIR CONDITIONERS, which protects the outdoor unit from direct rain impact and extends the device's lifespan. Learn more about this protection in the article Protecting the outdoor unit from rain and insects – why and how to do it.

Frosting of the outdoor unit

In heating mode, the outdoor unit naturally occasionally frosts over. The air conditioner has an automatic defrost cycle – this is normal. Problems arise if defrosting fails (sensor fault, issues with the defrost circuit) or if the outdoor unit is installed in a location where frost builds up too quickly (e.g., in a shaded area, under a dripping eaves). In such cases, an ice buildup can block the fan and cause a breakdown. If the outdoor unit is long-term covered with a thick layer of ice and automatic defrosting does not work, turn off the air conditioner and call a service technician.

Problem #7: Error codes – what your air conditioner is telling you

Modern air conditioners display error codes when a problem occurs – on the indoor unit display, on the remote control, or in the smartphone app. Each manufacturer has its own system of codes, but there are certain patterns. Here are the most common categories of error codes:

E1 / F1 and similar – sensor faults: The temperature sensor (inside, outside, on the heat exchanger) shows an error value or is disconnected. Sensors are inexpensive parts, but their replacement requires opening the unit.
E3 / H3 – high pressure: The pressure at the compressor outlet is too high. Typical cause: a clogged condenser in the outdoor unit, a blocked fan, or excessively high ambient temperature.
E4 / L3 – low pressure: The pressure in the system is too low. Typically: refrigerant leak or clogged pipe/expansion valve.
E5 – compressor current protection: The compressor draws too much current – mechanical compressor failure, power supply issue, short circuit.
P0 / C5 – communication error: The indoor and outdoor units cannot communicate. Cause: damaged communication cable, power outage, or control board fault.

Always note the code that appears and compare it with the device's user manual. You can also take a photo of the code – the service technician will appreciate it when you show it to them during the service visit, as it speeds up diagnosis.

Problem #8: Electrical and control faults

The control electronics of modern inverter air conditioners are sophisticated – the inverter (frequency converter) smoothly regulates the compressor speed and maximizes energy efficiency. With this sophistication, however, comes increased vulnerability.

Inverter failure

The inverter module is sensitive to overvoltage and undervoltage, to dusty environments, and to humidity. Inverter failure is indicated by the compressor not starting at all, the air conditioner operating only at full capacity (as if it were not inverter-based), or error codes related to the compressor appear. Replacing the inverter module is technically complex and not cheap – for older units, it is recommended to calculate the costs versus a new device.

Control board failure

The main control board (PCB) coordinates all functions of the air conditioner – it reads values from sensors, controls the fan, compressor, and valves. Its failure can manifest in many different ways: random restarts, ignoring commands from the remote control, error codes that remain lit without an obvious reason. PCB failure is a laboratory-level issue – sometimes it can be repaired by replacing a specific component, sometimes the entire board must be replaced.

When to call a service and when you can fix it yourself

This is a practical question every user faces. Here is a clear division:

You can handle it yourself (without certification):

Cleaning air filters (every 4–6 weeks)
Replacing batteries in the remote control
Checking and resetting the circuit breaker (if it tripped once and the device works afterward)
Removing large dirt from the outdoor unit (leaves, spider webs)
Checking that nothing is blocking the airflow in front of the outdoor unit
Installation of protective covers and accessories, such as the bottom cover for IVAR.2.0 or the bottom cover for IVAR.2.0 9HP MINI

Requires a certified technician:

Any work involving refrigerant (refilling, evacuating, leak detection)
Replacement of the compressor, inverter, or control board
Repair or replacement of refrigerant circuit piping
Electrical work on the air conditioner's power supply
Deep biological disinfection of the heat exchanger
Cleaning of the condenser heat exchanger using chemical agents

How to prevent malfunctions – preventive care

The vast majority of malfunctions can be prevented by regular maintenance. Practical experience shows that customers who perform a comprehensive service once a year have minimal problems with their air conditioning even after 10–15 years of operation. On the other hand, those who do not check their air conditioning at all start to have problems already after 3–5 years.

Basic annual service plan:

Every month: Check and clean air filters, visual inspection to ensure that nothing blocks the outdoor or indoor unit
In spring (before the summer season): Comprehensive chemical cleaning of the indoor unit, cleaning of the outdoor unit condenser, check of condensate drainage, check of refrigerant pressures, check of electrical connections, disinfection
In autumn (before the winter season): Visual inspection of the outdoor unit, check of protective elements (covers, grilles), test of heating and defrosting functions, check of the condensate system

For systems IVAR.2.0, we recommend using original accessories – an overview of all available parts and accessories can be found in the article Accessories for IVAR.2.0 air conditioners – overview of parts and their functions. Properly used accessories not only extend the life of the device but also simplify its maintenance.

If you are interested in the correct calculation of performance for your specific situation or space, see the article What air conditioning capacity do I need – calculation based on area and type of space – underpowered or overpowered air conditioning tends to fail much sooner because it operates under unsuitable conditions.

Typical scenarios from practice

Scenario 1 – Restaurant, air conditioning stopped cooling in the middle of summer: After inspection, we found that the outdoor unit was placed directly behind a kitchen extractor fan – the air entering the condenser had a temperature of around 45 °C instead of ambient 32 °C. The device was shutting down every 15 minutes for safety. The solution did not require any repairs – it was enough to redirect the extractor fan exhaust in another direction. Lesson: the placement of the outdoor unit is critical, more on this in the article Installation of air conditioning step by step – procedure and installation requirements.

Scenario 2 – Office, indoor unit dripping on the ceiling: The drain pipe was laid without a slope, even with a slight rise. Over three years, a biological layer formed in it, which blocked the pipe. Water had nowhere to drain and flooded the pan. Solution: cleaning and reinstallation of the drain with the correct slope. The cost of ceiling repairs (wet drywall) was higher than the entire service intervention. Lesson: proper installation saves nerves and money.

Scenario 3 – Family house, air conditioning humming and vibrating: After checking the outdoor unit, we found a squirrel nest directly in the cabinet. The squirrels damaged the insulation of the communication cable and contributed to the vibrations by loosening one of the fan mounting screws. The nest was removed and the damaged parts replaced. Preventive installation of insect protection kit for IVAR.2.0 would have completely prevented this problem. Lesson: insect protection is not a luxury, but an investment.

Scenario 4 – Showroom, air conditioning not heating in winter: The device had a cooling temperature limit set to –15 °C (factory setting), but the customer wanted to use heating in winter down to –20 °C. In reality, the unit was not dimensioned for such extreme conditions. Solution: combining air conditioning with another heating source during extreme cold. More on this combination can be found in the article Air conditioning in professional heating – combination with heat pump and heating.

Most frequently asked questions (FAQ)

Can an air conditioner explode or cause a fire?

Explosion of an air conditioner is extremely rare and is usually the result of a combination of several serious faults (leakage of refrigerant in combination with flammable refrigerant and an ignition source). Common refrigerants used today (R32, R410A) are non-flammable or only slightly flammable. A greater risk is fire from an electrical fault – therefore, never ignore the smell of burning, and if the air conditioner emits such a smell, disconnect it from the power immediately.

How do I know if refrigerant is leaking from the air conditioner?

Direct visual detection of a leak is almost impossible without specialized equipment – refrigerants are colorless and mostly odorless. Indirect signs of a leak: the air conditioner is cooling worse and worse even with clean filters, oil stains are visible on the pipe or connections (compressor oil leaks together with refrigerant), the indoor unit is frosting faster than usual, or you hear a bubbling sound in the pipe. Confirmation and localization of the leak is always done by a certified technician using an electronic leak detector or UV dye.

How much does a typical air conditioner repair cost?

Prices vary significantly depending on the fault and region. Roughly: service call and diagnosis 50–100 €, cleaning and maintenance 80–200 €, refrigerant recharge including leak detection 150–400 €, sensor replacement 80–150 €, control board replacement 200–600 €, compressor replacement 400–1 200 € (+ labor). For older units (over 10 years), if the repair cost exceeds 50% of the value of a new unit, it usually pays to invest in a new unit.

Can I use the air conditioner if it displays an error code?

It depends on the code. Some codes are informational (e.g. a service reminder) and the device continues to function. Other codes indicate active protection – the device shuts itself down. Never force the air conditioner to operate if it is actively displaying an error code related to refrigerant pressure, compressor temperature or electrical faults – you risk expensive secondary damage. Always note the code, find it in the manual and call a service if in doubt.

Must air conditioner servicing be performed by a certified technician?

It depends on what is meant by servicing. Cleaning filters, replacing batteries in the remote control, installing protective covers – these can be done by anyone. Any work involving refrigerant (recharge, discharge, leak detection) is subject to F-gas legislation and requires certification of both the person and the company. Violation of this obligation can result in high fines and also lead to the voiding of the device warranty.

How long will an air conditioner last without major repairs?

With regular annual maintenance and proper use, the realistic lifespan of a split air conditioner is 12–20 years. Without any maintenance, the first serious faults typically appear after 4–7 years. The compressor – the most expensive part – can last 15 years or more in a well-maintained unit with stable electrical supply. Control electronics are more sensitive and their lifespan depends on the quality of the power supply, environment and manufacturer.

Conclusion

Air conditioner faults are largely predictable and preventable. The key lies in regular inspection, cleaning filters, protecting the outdoor unit from physical damage and annual professional service. If a fault does occur despite this, the ability to recognize symptoms will help you resolve the situation faster and cheaper – either by fixing simple issues yourself or by communicating clearly with the service technician, who thus does not waste time on unnecessary diagnostics.

Remember that every ignored minor fault has a tendency to turn into a much more expensive one. A dripping condensate hose that you fix in 20 minutes of work will, if ignored, turn into a damaged ceiling costing several hundred euros. A bubbling sound in the pipe that you ignore means a dropping refrigerant level – and if you ignore it long enough, it leads to a compressor failure. A systematic approach to maintenance, proper accessories and occasional consultation with a specialist are the cheapest way to go.

Do you have a question about this topic?

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